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Session 86 - Coronal Activity.
Oral session, Thursday, June 13
Wisconsin Center,
A major limitation in the analysis of solar disk images is that only 2D information is observed. 3D coronal magnetic structures can be modeled by comparing coronal images and field extrapolations. If a good correspondence is found between loops in the X-ray image and those derived from the extrapolation, then the extrapolated 3D coronal magnetic structure can be used for information about the height of the X-ray features. We show that even the simplest 3D extrapolation, the potential extrapolation, can be useful for the analysis of observed X-ray loops.
For this analysis of 5 different active regions, we use Sakurai's potential field extrapolation code to determine the 3D potential model of the coronal magnetic structure from both Marshall Space Flight Center (MSFC) magnetograms and Kitt Peak magnetograms. The 3D magnetic field is compared to images of the persistent X-ray brightness derived from Yohkoh SXT images. Only some of the X-ray loops in some active regions fit well with 3D coronal potential magnetic structures. Large differences between the potential loops and observed loops that have one foot in the same place show that the observed loop traces nonpotential field. For many of the cases where there is no good fit, at least one footpoint of the observed loop is in a sizeable region of strong magnetic shear, so that potential coronal field would not be expected.
Many of the extrapolated 3D magnetic field lines are far from any bright X-ray loop. That is, the active region is filled with magnetic loops, but only a fraction of these strongly emit X-rays. Since not all of the coronal structures experience strong heating, some factor is controlling which structures do. We have also found from these active regions that the presence of a neutral line with strong magnetic shear is a favorable condition for strong heating. Large loops in the high coronal envelope of an active region are apparently selected for enhanced heating by the presence of such magnetic shear near a footpoint of the large loop, independently of whether or not the envelope field is strongly nonpotential.
